For the past four years experiments of several types have been performed to elucidate mechanisms by which metabolic pathways of bacteria evolve. Using the specialized catabolic pathways of Pseudomonas as easily studied models with great significance in the cycles of matter, these studies have ranged over the question of pathway delineation regulation, and overall genetic organization. Emphasis has been placed primarily upon direct experimentation, but some comparative studies of certain enzymes have been carried out using immunological techniques. A number of clear-cut hypotheses can now be subject to direct experimental test in our laboratories using techniques and biological material from the armamentarium that has been developed. In a search for the cause of unexpectedly high intensity of linkage among functional related genes for catabolic pathways in P. aeruginosa discovered earlier in our laboratories a study of the role of plasmids and the transfer of genes for catabolic pathways has been undertaken in the pseudomonads. It is hoped to be able to reconstruct the genetic basis for the characteristic nutritional phenotypes and explore the possible role of plasmids in the great nutritional versatility of the pseudomonads. The origin of the enzymes specified by the linked and functionally related genes for pathways of aromatic acid catabolism has been undertaken by immunological means to discover whether homologous enzymes from related organisms show relationships to one another interpretable on genetic or evolutionary grounds. A systematic study of the effects of functional stress placed upon a pathway whose nature and regulation are understood will be undertaken. Substrate analogues and amino acid analogues will be used to disturb the normal functional relationships in such a pathway in cells growing in continuous culture. The nature and temporal order of mutants that arise will be studied to obtain information on how metabolic pathways arise, undergo functional modification, and come to be regulated in bacteria.